1. Field of the Invention
This invention pertains generally to fixtures for inducing strain on a specimen in a transmission electron microscope, and more particularly to an apparatus and methods for inducing biaxial and radial strain on a thin film as a function of change in temperature.
2. Description of Related Art
Measurement of mechanical properties and the identification of deformation mechanisms from direct micro-structural observations of thin films has been done using various experimental techniques. Such techniques present challenging problems because bulk methods such as uniaxial tension testing, are very difficult to apply directly to films. Difficulties include generating forces and strains required in a small space, gripping the film, and prevention of bending force components in the film. While micro-electromechanical uniaxial strain fixtures have been fabricated, the approach does not generate the state of strains and stresses seen in chemical and biological micro-sensors, micro-actuators, passivation layers, micro-electronics, data storage and other film-based devices. The stress states that develop in these devices, whether intrinsic or extrinsic, are typically biaxial, plane-stress in nature. Bulge testing has been used to evaluate the biaxial modulus of thin films, but this method requires elaborate hardware to induce a state of stress that is generally biaxial but still varies across the bulged sample. In addition, it is infeasible to install the complex bulge test apparatus inside the five cubic millimeter volume and vacuum environment of a Transmission Electron Microscope (TEM) objective lens.
Inducing radial strain through direct thermal expansion for observation of a thin film by a TEM is infeasible. For example a stainless steel ring would need to be heated to a temperature of nearly 3,000 deg. C. to achieve a desired expansion, which is a temperature in excess of the melting point of stainless steel.
What is needed is an apparatus that applies uniform, variable biaxial strain on a thin film and is configured to fit within the dimensional restraints and environment of a TEM. An apparatus that applies and releases biaxial strain to thin films in cycles to simulate fatigue for failure analysis is further desirable. An apparatus that allows observation of strain on a freestanding film without a substrate support is also desirable.